There are many circumstances in the oil industry where it is desirable to cut into or through downhole tubular goods within a well. For example, in the course of drilling a well, the drill pipe may become stuck at a downhole location. This may result from "keyseating" or as a result of cuttings which settle within the well around the lower portion of the drill string. In order to remove the drill string from the well, it may be necessary to sever the drill pipe at a location above the stuck point. Similarly, it is often necessary to carry out downhole cutting operations during the completion or operation or abandonment of oil or gas wells. For example, it is sometimes desirable to sever casing or tubing at a downhole location in order to make repairs or withdraw the tubular goods from a well which is being abandoned or repaired. In most cases, the pipe is reusable. In other circumstances, it is desirable to cut slots, grooves or perforations in downhole tubular goods. Thus, it is a common expedient to perforate the casing and surrounding cement sheath of a well in order to provide fluid access to a hydrocarbon bearing formation. Similarly, it is sometimes desirable to perforate tubing in the completion or recompletion of a well.
Chemical cutters can be used to significant advantage in the application of chemicals to cut, sever or perforate downhole tubular goods. For example, U.S. Pat. No. 2,918,125 to Sweetman discloses a downhole chemical cutter which employs cutting fluids that react violently with the object to be cut with the generation of extremely high temperatures sufficient to melt, cut or burn the object. In the Sweetman procedure, halogen fluorides are employed in jet streams impinging on the downhole pipe to sever or perforate the pipe. The attendant reaction is highly exothermic and the pipe is readily penetrated. Examples of chemical cutting agents disclosed in Sweetman are fluorine and the halogen fluorides including such compounds as chlorine trifluoride, chlorine monofluoride, bromine trifluoride, bromine pentafluoride, iodine pentafluoride and iodine heptafluoride. The cutting agent in the Sweetman device is contained within a chemical container portion of the tool comprising a tubular body closed at its upper and lower ends with threaded connect subs. Each connector sub has a threaded counter bore terminating in a shoulder to which a rupturable shear disk is seated. The shear disks are held in place by means of externally threaded jam nuts. A pressure sub is located above the chemical section and contains a suitable explosive propellant. With ignition of the propellant material, sufficient gas pressure is injected to rupture the upper shear disk and thence the lower shear disk with the attendant displacement of cutting agent into an ignitor sub which contains suitable ignitor material such as sequential bodies of steel wool of progressively increasing coarseness and decreasing density. The cutting agent is then displaced into a discharge head where it is expelled from the tool through radial ports in jet cutting streams. In Sweetman, the cutting ports extend radially from a central bore within the discharge head of the cutting tool which terminates in a reduced diameter bore which is open to the lower or front end of the cutting tool. The reduced diameter bore is internally threaded to receive a threaded plug which closes the lower end of the bore.
Another chemical cutting tool is disclosed in U.S. Pat. No. 4,345,646 to Terrell. In this tool, a chemical module assembly is closed at its upper and lower ends by means of rupture diaphragms held in place by diaphragm retainers threaded into the chemical module sub. The rupture diaphragm is constructed with an area of reduced cross-section in order to facilitate rupturing at a specified pressure differential in order to more or less completely open the area of the diaphragm to facilitate the discharge of the chemical cutting agent.
Another chemical cutting tool is disclosed in U.S. Pat. No. 4,620,591 to Terrell et al. Here, the chemical sub assembly contains dual diaphragm seals at the opposed ends thereof. The dual diaphragm seals include upper and lower rupturable membranes which are separated by a dead air space. The dual diaphragm seal is held in place by means of an internal sleeve having external threads, which is threaded into place within the internal pin sections of the chemical sub.
As further disclosed in U.S. Pat. No. 4,619,318 to Terrell et al., objects may be perforated or in some instances, completely dissolved with no debris left in the well through the use of a downhole chemical cutter. As disclosed in this patent, the chemical cutting tool may be provided with a downwardly extended nozzle provided with a suitable stand-off sleeve. In addition to the halogen fluoride cutting agents as disclosed in the aforementioned patent to Sweetman, further cutting agents as disclosed in the Terrell et al. patent include nitrogen fluoride sources.
Yet another chemical cutting tool is disclosed in U.S. Pat. No. 4,494,601 to Pratt et al. Here, a lower part of the cutting head structure is open to well fluid and a piston plug is interposed immediately above the cutting ports. The cutting ports may be closed to the exterior of the well by means of an internal sleeve positioned in the bore of the cutting head immediately in front of the piston. As in the cutting tools described above, the cutting ports lie in a single plane perpendicular to the centerline of the tool.